1. Field of Invention
The invention relates to an image forming apparatus that forms an image by supplying and adhering toner onto a photosensitive drum on which an electrostatic latent image is formed to visualize the electrostatic latent image.
2. Description of Related Art
In a conventional image forming apparatus having a developing device that uses a non-magnetic one-component agent, toner is accommodated in a toner chamber. The toner is supplied to a developing roller, which carries the toner to a photosensitive drum. When the toner is supplied to the photosensitive drum, an electrostatic latent image formed on the drum is visualized as a toner image. When the visualized toner image is transferred onto a paper to be printed, characters or images are formed on the paper.
To form the electrostatic latent image on the photosensitive drum in such an image forming apparatus, the surface of the photosensitive drum is uniformly charged with the same polarity as the toner and then radiated with a laser beam, to selectively remove electric charges on the areas stuck by the laser beam. From a difference of potential between the electrostatic latent image formed and the toner on the developing roller, the toner is electrostatically attracted onto the electrostatic latent image and the toner image is formed.
To charge the photosensitive drum, conventionally a charging device for charging by use of high-voltage corona discharge is disposed adjacent to the photosensitive drum.
However, when a high voltage is applied to the charging device, the charging device electrostatically attracts airborne motes around.
In particular, as the charging device is disposed nearby the photosensitive drum, the motes that generate upon the rotation of the photosensitive drum are prone to be attracted to the charging device.
Further, if the charging device is covered with motes in excess, the balance of corona discharge is lost, which generates arc discharge on a part of the charging device. When the arc discharge is generated, the surface of the photosensitive drum is not uniformly charged. Accordingly, as a result of printing, a printout includes a black or white streak on the paper or the entire of the printout becomes black.
The invention provides an image forming apparatus capable of obtaining good results of image forming by eliminating contamination onto the charging device due to accumulated motes thereon.
As the invention provides a method to reduce or eliminate accumulation of motes on the charging device that charges a photosensitive member, the deterioration of image quality, due to improper charging as may occur in the conventional apparatus, does not occur. In addition, the contamination of the charging device can be prevented effectively, which can increase the operating life of the charging device. In general, a charging wire for the charging device is very thin and careful attention should be paid during cleaning to avoid cutting the wire. However, if the degree of the contamination to the charging device is low, as provided by the invention, the frequency of cleaning of the charging wire is reduced, so that the risk of damage to the charging wire can be reduced.
In an image forming apparatus according to the invention, charging amount control means control a charging amount of the charging device. The charging amount control means controls the charging amount to reduce the charging amount during a standby period from the charging amount used during an image forming period. In other words, when the charging amount from the charging device during the standby is reduced, motes that accumulated on the charging device during the standby can also be reduced. This is advantageous for prevention of improper charging.
To reduce the charging amount during the standby period, a voltage applied to the charging device during the standby period is controlled by chopping. Therefore, there is no need to provide a dedicated circuit for reduction of the charging amount. Further, as the on time and off time for the charging voltage in chopping control are equalized, the structure of the circuit can be simplified.
On the other hand, as an alternative to controlling the voltage to be applied to the charging device during the standby period by chopping, the contamination to the charging device can be prevented or reduced by lowering the charging current of the charging device during the standby period in a similar way.
A fan to generate airflow inside the image forming apparatus and airflow controlling means to control a quantity of the airflow generated by the fan may also be provided. Increasing the quantity of the airflow generated by the fan before the charging device starts charging is also advantageous to prevent or reduce the contamination to the charging device.
When the quantity of the airflow is increased before the charging device starts to charge, it is possible to generate the airflow required to eliminate or reduce airborne motes already existing in the image forming apparatus before charging, by ejecting the motes outside of the image forming apparatus. Thus, such motes, which already exist in the apparatus before charging and which are likely to be attracted to the charging device with the start of charging, are carried on the airflow and ejected outside the apparatus.
In particular, when the quantity of the airflow generated by the fan is controlled so as to increase as print data is received, the effects described above are further increased. Therefore, the charging device is not contaminated due to motes, and charging error does not occur. Thus, the deterioration of the image quality can be prevented or reduced.
Further, the power consumption and the noise of the image forming apparatus can be reduced by reducing the quantity of the airflow from the fan after the charging is completed. At this time, if the quantity of the airflow from the fan is increased for a fixed period of time, motes accumulated near the charging device can be effectively removed. The fan also emits the heat generated in a fixing unit or a control circuit outside the image forming apparatus, to keep the inside of the apparatus cool.